Leetcode 125：Valid Palindrome

Given a string, determine if it is a palindrome, considering only alphanumeric characters and ignoring cases.

Note: For the purpose of this problem, we define empty string as valid palindrome.

Example 1:

Input: “A man, a plan, a canal: Panama”
Output: true
Example 2:

Input: “race a car”
Output: false

C++
 bool isPalindrome(string s) {
        bool res = true;
        int i = 0;
        int j = s.size()-1;
        while(i <= j)
        {
            if(!isalnum(s[i]))
            {
                ++i;
                continue;
            }
            if(!isalnum(s[j]))
            {
                --j;
                continue;
            }
            if(isalnum(s[i])&&isalnum(s[j]))
            {
                if(tolower(s[i]) != tolower(s[j]))
                {
                    res = false;
                    break;
                }
                else
                {
                    ++i;
                    --j;
                }
            }
        }
        return res;
    }